This application claims the priority of 196 33 194.3, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a serial hybrid drive which can be used, in particular, in motor vehicles. In such hybrid drives, one or more electric drive motors make mechanical drive power available, for example for driving the drive wheels using the motor vehicle. The required driving energy is generated by the internal combustion engine/generator unit and, depending on the operating phase, fed directly to the electric drive motors or buffered in the energy store.
In most conventional motor vehicle serial hybrid drives, driving during specific operating phases, for example urban driving, occurs for minutes or hours using only the energy previously stored in the energy store, with the internal combustion engine remaining switched off. In order to provide such driving conditions, energy stores are used which have a relatively high storage capacity of, typically, more than ten kilowatt hours.
Laid-open Application EP 0 437 266 A2 describes a motor vehicle with serial hybrid drive in which the internal combustion engine/generator unit is controlled as a function of the load situation of the energy store and/or of the power output situation of the electric drive motor, for which purpose different variants are proposed. In a first known variant, the internal combustion engine is operated in a type of two-point operation optionally at a power speed selected with optimization in view, or at an idling speed.
In a second variant, the internal combustion engine is controlled to a higher or lower output power, and thus is approximately matched to the vehicle power requirement. In this case, the internal combustion engine is intended to run for as long as possible at or near its operating point of optimum consumption, with a fluctuation preferably of at most 10% being permitted.
In yet a third variant, the electric power output by the generator is controlled, and this represents a continuous matching of the generator power to the power output situation of the electric motor. In the two last-named variants, the internal combustion engine is preferably assigned a controller which selects for the respectively required internal combustion engine power a speed/torque combination which is as favorable as possible with regard to fuel consumption and/or pollutant emission. The remaining energy fluctuations are compensated in each case by the energy store, which is designed as a flywheel store.
DE 41 33 013 A1 discloses serial hybrid drives of which have a flywheel or accumulator for storing electric energy. Electric energy is fed into the respective electric drive motor optionally from the internal combustion engine/generator unit and/or the energy store as a function of the fuel consumption, exhaust gas quality, noise emission and/or the loading of the assembly of the internal combustion engine as well as the charge state of the energy store. The optimum operating point of the internal combustion engine is set, to the extent possible, as that which lies in a middle power range of the internal combustion engine in which the engine has a high efficiency.
In the event of a power requirement for the electric drive motor which has been increased in the short term, the internal combustion engine remains unchanged in its previous operating state, and the additional energy for the electric motor is made available by the energy store. If the increased power requirement of the electric drive motor lasts longer than the time interval variably prescribed, for example, by the charge state of the store, the internal combustion engine is accelerated in the conventional way by customary engine control measures.
An object of the present invention is to provide a serial hybrid drive which can be realized with a comparatively low outlay and permits the vehicle to be operated with as little pollution as possible.
The foregoing object has been achieved in accordance with the present invention by providing a serial hybrid drive having an internal combustion engine, a generator coupled mechanically to the internal combustion engine, an energy store coupled electrically to the generator, and at least one electric drive motor electrically connected to the generator and the energy store, characterized in that the internal combustion engine/generator unit is operated along an operating characteristic whose power corresponds to the temporally smoothed power requirement of the electric drive motor, the engine always being operated at or near full load, and the energy store has an energy storage capacity of at most a few kilowatt hours and a high power density and serves the purpose of compensating the short-term power differences between the instantaneous power requirement of the electric drive motor and the power output by the internal combustion engine/generator unit and corresponding to the temporally smoothed power requirement of the electric drive motor.
In the hybrid drive according to the present invention, the internal combustion engine/generator unit is always operated along an operating characteristic whose power corresponds to the temporally smoothed power requirement of the electric drive motor. The internal combustion engine always operated at or near full load and thus in a manner which is very low in pollutants and favorable in terms of consumption.
The recognition that the operating characteristic for controlling the operation of the internal combustion engine/generator unit should be based, not directly on the power requirement curve of the electric drive motor, but on its curve as smoothed temporally in accordance with any one of the customary methods produces the significant advantage that there are no short-term sudden power variations in the operation of the internal combustion engine. Instead, the I.C. engine is subjected only to slow temporal changes, with the result that the engine produces minimum pollutant emissions, i.e. the engine can be operated in accordance with the temporally smoothed drive power requirement.
The xe2x80x9ccalmedxe2x80x9d internal combustion engine operation, in which there are no sudden changes in throttle valve angle, prevents peak values in HC concentration and CO concentration in the exhaust gas. By comparison with conventional vehicle drives, this operation effects a markedly more precise adherence to the stoichiometric air/fuel ratio so as to improve the rate at which the catalytic converter operates.
The internal combustion engine can be operated in the engine operating map area which is most favorable in terms of emission, and the calmed engine operation in the restricted engine operating map areas keeps the exhaust gas temperature window small and avoids peak values in the exhaust gas temperature. All of this benefits both the rate at which the catalytic converter operates and its durability.
The short-term power differences occurring between the instantaneous power requirement of the electric drive motor and the calmed power output of the internal combustion engine/generator unit are compensated by the energy store which, for this purpose, has a high power density. The energy store is sized in terms of its storage capacity to at most a few kilowatt hours, and is thus has a substantially lower capacity than conventional hybrid drive stores. Consequently, the energy store can be very compact and can be realized, for example, as a flywheel or super-condenser. Thereby, the energy store has a storage capacity of at most a few hundred watt hours.
In a serial hybrid drive developed according to the present invention, internal combustion engine is operated exclusively at full load. The instantaneous operating point is set on the full-load characteristic as a function of the generator load. This mode of operation allows the internal combustion engine to be used as a spark-ignition engine which has no throttle valve with associated controller. The internal combustion engine can therefore be realized in a comparatively simple and cost-effective way.
Because of the lack of throttle valve movements achievable with the present invention, the stoichiometric air/fuel ratio can be observed precisely at any time. Consequently, short-term peaks in pollutant emission and associated drops in the efficiency of the catalytic converter can be prevented. The internal combustion engine gains optimum efficiency because of being completely without a throttle valve, and is operated with favorable HC raw emissions. Thereby other, conventional measures such as direct injection or completely variable valve gear are unnecessary for this purpose.
A serial hybrid drive developed according to another aspect of the present invention, includes a catalytic converter which is heated electrically before the cold start of the internal combustion engine by the energy from the energy store which, with its high power density, is ideally suitable for very quick heating of the catalytic converter.